Problem:  Strange error when heredoc marker starts with "trim".
Solution: Check for whitespace after "trim" or "eval" (zeertzjq)

For :python3 etc., a heredoc marker that starts with a lower-case letter
is valid, and when it starts with "trim" it works in a script but not in
a function, and this PR makes it works in a function.
For :let, a heredoc marker that starts with a lower-case letter is not
valid, but when it starts with "trim" or "eval" the error can be a bit
confusing in a function, and this PR make it less confusing.

closes: vim/vim#16574

https://github.com/vim/vim/commit/449c2e5454735fe1cfc8c21b2c6880d6bdf4cd6e

Problem:    Heredoc in scripts does not accept lower case marker.
Solution:   Allow lower case only in non-Vim scripts. (Ken Takata,
            closes vim/vim#6019)

https://github.com/vim/vim/commit/6ab0953fefe31fef91e40752a675ceb60fc2fe03

Problem:    Heredoc for interfaces does not support "trim".
Solution:   Update the script heredoc support to be same as the :let command.
            (Yegappan Lakshmanan, closes vim/vim#5916)

https://github.com/vim/vim/commit/6c2b7b8055b96463f78abb70f58c4c6d6d4b9d55

Problem:    Not all code for options is tested.
Solution:   Add more tests. (Yegappan Lakshmanan, closes vim/vim#5913)

https://github.com/vim/vim/commit/1363a30cef382b912bf092969e040333c5c293c6

The new oldtest directory is in test/old/testdir. The reason for this is
that many tests have hardcoded the parent directory name to be
'testdir'.